The present invention relates generally to the bulk separation of gaseous components generated during coal gasification, and more particularly to the efficient and cost-effective separation of discrete components of such coal gasification products especially hydrogen at high purity levels and acid gases by pressure swing adsorption.
Coal gasification is receiving increased interest as a suitable mechanism for the production of high purity hydrogen and other valuable fuel gases including carbon monoxide and methane. In coal gasification processes steam and oxygen are utilized in the gasifier for the production of hydrogen-rich product gases. A water-gas shift reaction is then used to provide high purity hydrogen from the product gases. The hydrogen so produced is usually separated from the fuel gases CO and CH.sub.4 and the acid gases (CO.sub.2 +H.sub.2 S) by distillation or other vapor-liquid separation techniques. The removal of the acid gases and other gaseous sulfur-bearing components present in the coal gasification products is normally achieved by low-temperature, physical and chemical adsorption apparatus. These processes are cryogenic and are high energy users which render the process relatively expensive for hydrogen production.
Pressure swing adsorption is a relatively recent development for gas separation. Generally, pressure swing adsorption provides for the selective concentration or adsorption of one or more of the gaseous components (adsorbates) of a gaseous mixture on the surface of porous solid adsorbents such as zeolites, alumina, activated carbon, and molecular seive carbon. The adsorption of the gas components is in accordance with their molecular makeup and as defined by Van der Waals equation, certain gas components are more readily adsorbed on adsorbents at particular pressures. For example, CH.sub.4 is adsorbed by activated carbon at a lower pressure than hydrogen and has a greater retentivity than hydrogen so as to remain on the adsorbent when the partial pressure of the adsorbent bed is reduced. This desorption provides for the separation of the gas components. Details of pressure swing adsorption and other gas adsorption processes are discussed in the publication: "Gas-Adsorption Processes: State of the Art", in "Industrial Gas Separations" by George E. Keller, II, American Chemical Society, Washington, DC (1983) pp. 145-169.
Pressure swing adsorption processes have been used where trace amounts of impurities are removed from a product gas, or for air drying. In pressure swing adsorption processes the partial pressures of the adsorbates on the adsorbent are utilized for the selective separation of a desired gaseous component from a gaseous mixture. Often a portion of the less adsorbed product is utilized to purge the adsorbent of the gases remaining in the adsorbent-containing column. Thus, an inevitable feature of the known pressure swing adsorption processes is that a portion of the less adsorbed product is lost to the purge stream during the process. This particular problem provides a strong implication that the purge stream can not be of high purity so as to limit known pressure swing adsorption processes to operations where only one product of high purity is desired.
The pressure swing adsorption process as previously known have not been suitable for bulk separation of the multi-component gases produced by the gasification of coal in concentrated or high purity form. All of the gaseous components produced by coal gasification are considered to be valuable products by themselves especially if recoverable at high purity levels.